<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>javascript</title>
  <style>
    * {
      margin: 0;
      padding: 0;
      box-sizing: border-box
    }
    body {
      font-size: 14px;
      padding: 20px;
    }
    ul li {
      line-height: 30px;
    }
  </style>
</head>
<body>
  <ul>
    <li></li>
  </ul>
</body>
<script src="https://cdn.bootcss.com/vue/2.6.8/vue.js"></script>
<script>
  var str = new String('hello')
  console.log(typeof str)
  console.log(typeof str.valueOf())
  console.log(new Boolean())
  var arr = [[1],[2],[3],[4]]
  var a =  arr.reduceRight((prev, curr, index, arr) => {
    console.log(prev, curr, index)
    return prev.concat(curr)
  }, [])
  console.log(a)
</script>
</html>